Add existing user to a group

I can never seem to remember how to do this without having to search. There are way to many programs that allow users and groups to be created/updated/modified.

Add an existing user to an existing group:

# usermod -a -G <group> <user>

Or for a list of groups:

# usermod -a -G <group1,group2,...> <user>

I found this at http://www.cyberciti.biz/faq/howto-linux-add-user-to-group/